1. Technical field
The present invention relates to a photovoltaic power generation controller and a power evaluation method in photovoltaic power generation control.
2. Related Art
In order to take power efficiently from a solar cell, the solar cell always needs to be made to operate at the maximum power point (MPP). Therefore, a general photovoltaic power generation system is provided with a maximum power point tracking control circuit (MPPT).
Many types of MPPT methods have already been reported, and one of those is a control method referred to as a hill-climbing method. In this control method, an oscillation component (hereinafter referred to as a “ripple component”) with a constant variation width is given to an operating voltage of a solar cell at a low frequency, using a power converter such as a chopper circuit; and the resultant inclination of power is calculated; and the operating point is moved to MPP. This control method has high adaptability with respect to changes in the external environment, and so is utilized widely.
In this hill-climbing method, improvement in the tracking speed to reach the MPP and output power oscillation suppression after MPP convergence have a trade-off relationship. An adaptive hill-climbing method in which: a variation width is automatically adjusted to have the optimum value to realize prompt tracking to MPP; and at the same time, oscillation in the vicinity of MPP is suppressed, has been proposed to overcome the above problem (Takahara, Yamanouchi, Kawaguchi, “Maximum Power Acquisition Control of Photovoltaic Power Generation System with Adaptive Hill-Climbing Method,” Trans.Inst. Elect.Engnr. Jpn. D, vol. 121, no. 6, pp. 689-693, 2001—non-patent document 1). In hill-climbing methods including this adaptive hill-climbing method, the variation width needs to be made as small as possible in order to raise MPPT accuracy after convergence at MPP. However, when conducting a hill-climbing method using a microcomputer (hereinafter referred to as an “MC”), a variation width for a ripple component needs to be determined in consideration of the resolution of an AD converter (hereinafter abbreviated as “ADC”). Many ADCs mounted in inexpensive MCs each have a low resolution and much internal noise, and have no expected accuracy. Therefore, a high-solution ADC is required to decrease variation width to raise the MPPT accuracy of after MPP convergence in the hill-climbing method, but this leads to an increase in product cost.
Meanwhile, in many MPPT control circuits not only employing a hill-climbing method but also other methods, MPPT accuracy deteriorating during periods of low solar radiation is known. In general, when selecting parameters for a control circuit, those parameters are selected in accordance with the maximum rating of a solar cell to be used. However, the output of a solar cell varies greatly with changes in the external environment, so the signal level input to the control circuit is reduced greatly during periods of low solar radiation. Therefore, control accuracy deteriorates due to ADC resolution problems, etc.
In order to detect minute output change by means of an ADC with a low resolution, detecting a voltage and a current of a solar cell with high gains is sufficient. However, when gains are increased, not only ripple components of the current and voltage but also the direct-current components thereof expand, leading to the problem of saturation in a measuring system.
From the above, with attention focused on the feature of a hill-climbing method in which MPPT is performed by evaluating a power change deriving from a ripple component, raising a gain of only the ripple component remaining after removing the direct-current component can be considered. However, when power is calculated from the current and voltage from which direct-current components have been removed, the resultant power completely differs from an original power value generated by a solar cell.
The inventors of the present application found conditions under which MPPT control can be performed accurately even in the case of removing direct-current components, and also found that by employing those conditions, correct power evaluation can be conducted, in the vicinity of MPP, only with ripple components even in the case of completely removing direct-current components. Only ripple components remaining after removing direct-current components reduces the risk of saturation of an amplifier, so the gain of the ripple component can be raised very high, whereby a minute power change can be detected with an ADC having a low resolution. By utilizing this, the accuracy of MPPT can be raised, and the accuracy during periods of low solar radiation can be improved.
Conventional MPPT control techniques in photovoltaic power generators are disclosed in, e.g., JP2005-070890 A (patent document 1) and JP09-091050 A (patent document 2). However, those techniques do not include a technique, like the one described above, in which: direct-current components are removed or suppressed in the vicinity of MPP; direct-current ripples are used; and switching gains are switched, thereby attaining an increase in accuracy of MPPT and an improvement in the accuracy during periods of low solar radiation.